This application claims the benefit of U.S. Provisional Application Ser. No. 60/538,054 filed Jan. 21, 2004 and is a Continuation-In-Part application that claims the benefit of U.S. Continuation application Ser. No. 10/360,237, filed on Feb. 6, 2003, now issued as U.S. Pat. No. 6,906,279 on Jun. 14, 2005, which claims the benefit of application Ser. No. 09/902,378 filed on Jul. 10, 2001, now issued as U.S. Pat. No. 6,576,859 on Jun. 10, 2003.
This invention relates to a weld gun for resistance welding metal components, such as fasteners, to a metallic object, such as sheet metal. More particularly, this invention relates to a method of using information obtained from a pin position sensor during the welding operation.
Weld gun assemblies are used to resistance weld fasteners to metallic objects such as body panels for vehicles. A fastener, such as a weld stud or nut, is loaded onto one of the electrodes. The electrodes are moved together with an actuator thereby placing the fastener into engagement with the panel under pressure. Very high currents are applied to the electrodes, which welds the fastener to the panel.
The use of weld guns is typically an automated process in which the fasteners are loaded into the weld gun assembly. Occasionally, the fastener is not properly loaded into the weld gun or the fastener may not be loaded into the weld gun at all. Under these circumstances, it is important to detect that there is a problem with the fastener so that the fastener can be properly welded to the panel. To this end, sensors have been used in an effort to determine the presence and orientation of the fastener in the weld gun. Typically, a linear variable displacement transducer has been connected to an electrode arm to measure the movement of the arm. If no nut is present the arm will move a greater amount than if a fastener is present and in the proper orientation. If the fastener is not in the proper orientation, then the electrode arm will move slightly less than if the fastener is in the proper orientation. In this manner, fastener presence and orientation has been detected.
It has been difficult to accurately detect the proper orientation of the fastener for several reasons. First, the position of the electrode arm does not always correspond to whether the fastener is in the proper orientation. That is, the location of the position sensor cannot detect fastener orientation accurately because of deflection in the weld gun, tolerance stack-ups and variations in the fastener. Second, the high electrode currents introduce inaccuracies into the position sensor signal. Even with magnetically shielded cables, enough interference from the electrode current is introduced to give false fastener readings. Applicant's U.S. Pat. No. 6,576,859 presents one solution to the problems of the prior art by measuring the pin position.
Prior art weld gun assembly configurations typically make comparative measurements that do not take into account work piece height variations or electrode wear. For example, metal thickness, waviness, projection height variation, or work piece deformation caused by prior cycling of the welder without current are all information ignored by prior art systems. Prior art systems needlessly extend the cycle time to account for variability in the above parameters. In applications where fasteners having small projections are used, the variability in work piece height can result in tight process windows or an inability to make a quality determination.
Prior art systems have monitored fastener upset using pressure to provide adaptive feedback. Upset is the collapse of the fastener projections during the welding operation. Displacement measurements using pressure have not been accurate enough to provide the required accuracy and dynamic response for a feedback control useful in the industry.
What is needed is a weld gun assembly that is capable of monitoring work piece height variations, fastener upset, and other welding operation parameters to reduce cycle times and provide improved feedback and control over the welding operation.